Various prior systems provide information about the relative positions of aircraft in a particular airspace.
One system has ultra-precise, airborne clocks aboard each aircraft to provide precise synchronizing reference signals, and each aircraft is prompted to simultaneously transmit an identification signal. Distance between aircraft is determined by measurement of the elapsed time between the transmission of an identification signal and receipt of that signal. This system provides only distance information and does not provide relative bearing or position information. It is readily appreciated that maintenance of all of the ultra-precise, airborne clocks is difficult. All of the aircraft within an airspace transmit identification signals at the same time, causing interference and confusion so that the ability of this type of system to resolve one signal from a large number of other signals is limited.
Another system uses a cooperative interrogation and reply technique whereby an interrogating aircraft transmits a signal and measures the elapsed time between the transmission of the signal and the receipt of a reply signal from a responding aircraft. The ability of the system to resolve the signals from each of a large number of aircraft occupying the same airspace is also limited.
A system whereby each aircraft carries its own expensive scanning radar system obviously presents problems of economics, not to mention the severe clutter and jamming caused by a large number of such systems operating within the same airspace.
Another system uses ground-based radar for determining the position of all aircraft within an airspace and transmits positional data to each aircraft by means of ground-based data transmission links. This system requires radar and data processing facilities which are not available in every geographic area.
Many of the prior location systems have nearly simultaneous signal transmissions from all the aircraft within a given airspace and create serious clutter and jamming problems when a large number of aircraft are present. It is recognized that conventional automatic direction finder systems and range measurement systems, such as the well-known VOR-DME and VOR-TACAN systems, enable an aircraft to accurately know its own angular bearing, or azimuth, and range with respect to a reference location. Many hyperbolic navigation systems, such as LORAN and DECCA, as well as various satellite-based systems, provide individual aircraft and other aircraft with their own positions. However, these systems do not provide an aircraft with information about the position of other aircraft in the same airspace so that collisions may be avoided.
One prior art system designed to overcome many of the problems of the prior art can be found in U.S. Pat. No. 4,380,050, issued to Tanner. The Tanner patent is directed to a collision avoidance system that assigns time slots based on the relative azimuth and range of an aircraft with respect to a local ground-based VOR station. Each aircraft transmits a signal identifying its relative position with respect to the location of the VOR station. The system disclosed in the Tanner patent effectively avoids collisions, but requires other aircraft to convert the relative position signals into global position information such as latitude and longitude.